1. This invention relates to an exhaust system for an internal combustion engine and, more particularly, to an improvement in detecting deterioration in function of a catalytic convertor of an exhaust system.
2. Description of Related Art
An exhaust system for an internal combustion engine typically has a feature for detecting deterioration of a gas purification means, such as a catalytic convertor, disposed in an exhaust system. This kind of exhaust system includes an oxygen sensor for detecting a concentration of oxygen in exhaust gases. A deterioration in function of the catalytic convertor is detected based on the fact that the catalytic convertor lowers emission levels of hydrocarbons (HC) in exhaust gases with an efficiency which is directly proportional to oxygen adsorbability of the catalytic convertor. The oxygen sensor is disposed in an exhaust line downstream from the catalytic converter so as to detect an oxygen level in exhaust gas; this oxygen level is representative of an efficiency of oxygen adsorbability of the catalytic converter, i.e., the degree of deterioration oxygen adsorbability of the catalytic converter. Such an exhaust system is known from, for instance, Japanese Unexamined Patent Publication No. 63-97,852.
Some V-type internal combustion engines have an air-fuel ratio sensor installed in an exhaust line for each group of cylinders in order to perform feedback control of an air-fuel ratio for each cylinder group. For precisely feedback controlling an air-fuel ratio, the V-type internal combustion engine is equipped with an oxygen sensor for detecting a concentration of oxygen in exhaust gases in order to detect deterioration of a catalytic converter installed in the exhaust line. Such a V-type internal combustion engine is known from, for example, Japanese Unexamined Patent Publication No. 61-138,849.
Because the air-fuel ratio feedback control is conducted independently for each of the two cylinder groups, a change in phase of an air-fuel ratio from rich to lean or vice versa occurs randomly between the two cylinder groups, due to differences in exhaust stroke between the cylinder groups, differences in properties and characteristics of various associated elements and sensors between the cylinder groups, and/or circumstances in which exhaust gases are introduced into the catalytic converters installed in the respective exhaust lines. This leads to inaccurate detection of deterioration in function of the catalytic converters.